Diffusion of Krypton and Xenon in Uranium Mononitride; a Density Functional Theory Study

Uranium mononitride is a strong candidate for an advanced nuclear fuel. In this work we use density functional theory to model the diffusivity of matrix U atoms, as well as the noble gas (Ng) fission products Kr and Xe, at the atomic scale under three stoichiometric conditions (U-rich, stoichiometric, N-rich). U self-diffusion is found to be dependent on stoichiometry, being largest under N-rich conditions. The U formation entropy significantly affects the U self-diffusion coefficient, indicating that it is necessary to consider the vibrational properties of the system to accurately describe diffusion properties in UN. The calculated Kr and Xe diffusion coefficients from the U vacancy (VU)-assisted mechanism are much larger than by interstitial mechanisms under the three stoichiometric conditions studied. The two mechanisms show opposite stoichiometric dependence, with the former increasing from U-rich to N-rich conditions and the latter decreasing. Kr moves significantly more quickly than Xe via the interstitial mechanism due to the larger atomic radius of the latter, while the effect of atomic size on the VU-assisted mechanism is negligible, indicating similar diffusivity of Kr and Xe in UN. The good agreement with experiment of our calculated Xe monovacancy-assisted diffusion coefficient indicates that the monovacancy-assisted mechanism governs the Ng diffusion in UN, and supports the accuracy of our theoretical model.